Segment 4 of Learning Module 11, and we were talking about Ardipithecus ramidus and how there's quite a large fossil assortment associated with this species that gives us lots of information. And here you can see the full skeleton, the 40% complete skeleton, which by the way, you actually don't need 100% of a skeleton to actually have a full skeleton. I know that doesn't really make sense, but...
The cool thing about humans is that we're very nicely symmetrical and so you actually don't need the whole skeleton. You only need the right part of the skeleton or the left, right? So you only need one side of the skeleton and then you actually have a full and complete skeleton. So it kind of depends on how much of this 40% or where that 40% is located that can actually give you a whole lot more of the skeleton than just 40%. So looking at this artistic rendering of Ardipithecus, what do you notice right away?
Look how long the arms are and look at the feet, right? The feet have this divergent toe, like we see just like in chimpanzees. And I always get students asking me, wait, wait, wait, wait, wait, wait. Okay.
So if Ardipithecus is in fact bipedal, Why is it that we see that they have a divergent toe, right? Because we talked about how one of the morphological adaptations for bipedality is that the toes are in line to provide good support and to provide balance. And that divergent toe is really only necessary if you need prehensility in your feet, which is what you would use in a arboreal environment for going around in trees, right?
So the question is, when you look at a trait, And I hope you will keep this in mind when you're looking at different kinds of traits, is that there should always be a question in your mind. Does this species have this trait because it is adaptive or is it because of ancestry? So you always look at a trait and you ask, is it adaptive or is it ancestry?
Because the two are not the same thing. In looking at the divergent toe, We see that the divergent toe of artepithecus is not adaptive for bipedality. So of course, if you look at this toe and you would go, well, what's going on here?
Is artepithecus not really bipedal? Are they kind of half bipedal? Are they half quadrupedal?
Are they not, you know, walking around all the time? Are they kind of doing a half bipedality thing? And we've already discussed in the last learning module, when we talked about bipedality, that there is no adaptation for something in between quadrupedality and bipedality.
Sort of a maybe like that crouched over type of bipedality that would be sort of this halfway point between quadrupedality and bipedality did not exist. It didn't exist because it's not an efficient means of locomotion. So the question becomes why, if it's not adaptive, why does Ardipithecus have a divergent toe?
And the answer has more to do with the question. with ancestry. It has to do with the fact that bipedality is a recent adaptation. It's a recent adaptation for Ardipithecus. This ultimately shows a transitional state between quadrupedality and bipedality.
So Ardipithecus is fully bipedal, and we're going to see that there's, there are lots of indications of that, but they have the retention of that big toe. because it is a recent adaptation. In other words, bipedality is so recent that they're in transition. They're moving from quadrupedality to bipedality and that big toe, it will eventually come in line over time.
Okay, looking at the cranial evidence of artepithecus, once again, we are looking at the foramen magnum. Now the foramen magnum, we mentioned already, the foramen magnum is a hole in the cranium that's in the back of the cranium in quadrupeds. And this is where the spinal cord meets up with the brain. And of course, it would need to be in line with the vertebral column, right?
So for example, in humans, the... Forman magnum is located directly under the cranium because it needs to be in line with the cervical vertebra, the neck vertebra, that, you know, as a biped, your spinal column goes all the way down directly underneath. the cranium.
In a quadruped, like we see with chimpanzees, they have to have this hole in the back of the cranium. Because if it was in the middle of the cranium, like you see with humans, they would not be able to raise their head, it would it would it would prevent them from even raising their head, because it wouldn't be in line with the rest of their vertebra. It needs to be in the back of the cranium so that they have actual head movement. Okay.
So it needs to be in line with the rest of the vertebral column that would extend horizontally from the cranium. Okay, so the location of the foramen magnum provides an excellent indicator of quadrupedality or bipedality. In artepithecus, we see that the foramen magnum is located right about here. If this was a chimpanzee, the foramen magnum would be located back here. And if it was a human, the foramen magnum would be located right about here.
So in other words, the foramen magnum is much further forward than you see in a quadruped, but not quite where it would be if it was a human. And the question becomes, well, why is this? Why is the foramen magnum in this middle place?
Is there some sort of half bipedality going on here? We've already established it doesn't work, right? There is no such thing as a half quadruped, half biped. And the only reason why the foramen magnum would change in position from the back of the cranium would be if artepithecus was a habitual and obligate biped.
So the question is, why is it not exactly in the location that we see with humans? And the answer is exactly... the same as what we see with the divergent toe.
As you look at this and you go is it adaptive? Is it adaptive to have the foramen magnum further forward but not all the way where it needs to be as a biped? No, there's no adaptive reason for that. It is ancestry. In other words the foramen magnum is in a place of transition.
It's in a a transitionary state from being a quadruped to a biped. It's not so transitional that you would have like this half bipedality thing. It is simply, it's almost there, but not quite. It's not affecting the artepithecus'ability to be bipedal. It just shows that it's almost there.
It's in a state of transition. Looking at the face, we can see that the prognathism in the front of the face, this is a view of the... Cranium looking above and from above and you can see here's the face and it sticks up quite a lot So very prognathic face that is a primitive trait.
We also see that Ardipithecus has a flattened cranial base in a human right we recall that the cranial base is much more rounded and If we ask once again, okay, why is Ardipithecus not have a rounded cranial base like humans if they're bipedal. And, and again, this shows a state of transition, right? That because bipedality is a recent adaptation, this is they still have the retention of this trait.
Furthermore, we don't see any increase in brain size. So a lot of the reason why we have roundedness on the bottom here isn't just because of balance, but because we have such a large brain, and that oval configuration, or I'm sorry that rounded configuration is really best for balance on the cervical vertebra. So we still have a relatively small brain going on here, pretty much the size of a chimpanzee, and we don't need this roundedness back here as of yet. So we're going to see that this flattened cranial base hangs on for quite some time in hominids.
There is one modern trait of artepithecus that we need to mention. Thin enamel caps on the molars. In chimpanzees, they have very thick enamel on their molars and their teeth in general.
Because they are chewing on such a coarse diet, if they had thin enamel like ours, their teeth would break. And so they need that thick enamel because, let's face it, there are not very many dentists in... equatorial rainforest of Africa and so you would need the thick enamel to protect the teeth in Chewing on very coarse foods in humans on the other hand we have very thick thin enamel on our teeth. In fact, this is a reason why we are very susceptible to cavities and such because we have thin enamel. And so the reason for that is because we have a much softer diet.
We don't need that really thick enamel. We're not chewing on some really coarse foods. All right, so looking at this evidence for artepithecus, we also need to cover the postcranial evidence.
The humerus, which is the upper arm bone of the arm, indicates that Ardipithecus is not weight-bearing, which means bipedality. We talked about how in gorillas and in chimpanzees, there are modifications to the joints of the hand and the elbow and the wrist and the shoulder, all showing load-bearing ability, which is necessary if you're a quadruped. Not necessary if you're a biped. So the upper arms are not being used for weight bearing at all, just like you see in humans, further proof that artepithecus is fully bipedal.
The legs also indicate obligate bipedalism with the angle of the femur and modification of the knee and so forth. We can also see with this complete skeleton things like body size, and there's an estimated body weight of artepithecus of 93 pounds. That's right in the range of chimpanzees. If you recall, chimpanzees are between 80 and 100 pounds. And so Ardipithecus is right in that range.
Essentially, Ardipithecus is like a chimpanzee that's walking around upright all the time. Pretty much most features of the body are like a chimpanzee, just fully bipedal, which is really, really fascinating to think about. So ultimately when we look at once again our cladogram here our evolutionary tree you can see that the once again that the monkeys break off from The the large bodied hominoids they have large body hominoids have their own bridge the small body hominoids Here's the the gibbons and the sammings the lesser apes. They have their own little branch then the large bodied hominoids There's the Asian form ring of tangs are the product today and then the african forms all the way up here you can see the modern gorilla chimpanzee and humans and then here's the human line up here at the very top and artepithecus is basically right about here right at the beginning of the hominid line so pretty recent in time and Basically, overall, looking at these early hominids, what's interesting about these three sites of the early hominids, Tujan Hills, Aramis, and Torres Manala, is that the environments in place at the time of these three early hominid species was predominantly forested land.
Most of the later hominid sites are located in areas that used to be very savanna-like land. So perhaps we're seeing at these early sites the very beginnings of human divergence from the large-bodied hominoids that were primarily arboreal and maybe the presence of a savanna-like environment wasn't what ultimately pushed for bipedality. These heavily forested areas may have been more likely to become marshland in climatic changes and that may have been an even bigger factor in the development of bipedality than even the savanna type environment.
We're going to end this segment here. In the next segment we are going to start talking about Australopithecus, which is another genus that's a little bit more recent in time, still classified as an early hominid, but we're going to see a few changes popping up here at the genus Australopithecus. All right, so please join me for segment five. We're going into segment five.